Field of the Invention
The present invention relates to an exposure apparatus, an exposure method, and a device manufacturing method.
Description of the Related Art
When a microdevice (a semiconductor element, a liquid crystal display element, or the like) is manufactured in a photolithography process, an exposure apparatus which exposes a substrate by projecting the pattern of a mask on the substrate is used. In order to project the pattern of the mask on the substrate precisely, the exposure apparatus is required to perform focus alignment (focus calibration) between the mask and the substrate accurately. One example of focus calibration methods includes a TTL (Through The Lens) method which measures the relative position of the substrate with respect to the mask and the focus position of the pattern of the mask via a projection optical system.
FIG. 8 is a schematic view showing an example of a conventional exposure apparatus having a focus calibration function in the TTL method. An example of focus calibration in the conventional exposure apparatus shown in FIG. 8 will be given. In order to illuminate a mask-side reference mark arranged on a mask 2 or mask stage 3 by an illumination optical system 1, a main controller 7 issues a command to a mask stage controller 8 and moves the mask stage 3. The main controller 7 issues a command to a substrate stage controller 11 and moves a substrate stage 6 such that a substrate-side reference mark 9 on the substrate stage 6 is arranged in correspondence with the mask-side reference mark. The main controller 7 drives the substrate stage 6 finely in the Z direction. A processor 12 calculates a coordinate position Z0 at which a detected light amount reaches its peak. FIG. 9 shows the relationship between the detected light amount and the coordinate position in the Z direction at this time. The position Z0 at which the detected light amount reaches its peak is obtained when the mask-side reference mark and the substrate-side reference mark 9 are in a conjugate positional relationship. A focus position is calculated by searching for the maximum value of that light amount. The processor 12 transmits calculated focus position information to the main controller 7. The main controller 7 can match the focus position of the pattern of the mask 2 with a substrate 5 by issuing a command to the substrate stage controller 11 and driving the substrate stage 6 in the Z direction by the shift amount of the focus position.
Japanese Patent Laid-Open No. 4-348019 discloses a method of calculating a focus position by illuminating a substrate-side reference mark, receiving reflected light of an image of a mask-side reference mark with a projection optical system via the projection optical system and the substrate-side reference mark, and detecting a change in that light amount.
When measuring the focus position, however, both the positional shift between the position of the substrate-side reference mark 9 in the X and Y directions and the projection position of the measurement pattern (mask-side reference mark) of the mask in the X and Y directions, and substrate-side telecentricity of a projection optical system 4 exist. Telecentricity refers to a magnification error with respect to the depth direction of an object. In this case, the measurement value of the focus position is deviated. In FIG. 10A, 100a to 100c indicate ideal states A without any positional shift and telecentricity. In FIG. 10B, 200a to 200c indicate states B in which both the positional shift and telecentricity exist. Each of 100a of FIG. 10A and 200a of FIG. 10B indicates a state (Best Focus state) in which the focus position of the mask-side reference mark matches a position a of the substrate-side reference mark 9 in the Z direction. Each of 100b of FIG. 10A and 200b of FIG. 10B indicates a state (+Defocus state) in which the position of the substrate-side reference mark 9 in the Z direction is at a position b shifted in the plus direction with respect to the focus position of the mask-side reference mark. Each of 100c of FIG. 10A and 200c of FIG. 10B indicates a state (−Defocus state) in which the position of the substrate-side reference mark 9 in the Z direction is at a position c shifted in the minus direction with respect to the focus position of the mask-side reference mark. As shown in 200a of FIG. 10B, the state B indicates that the projection position of a measurement pattern in the X direction in the Best Focus state is shifted with respect to the center of the substrate-side reference mark 9 in the X direction, that is, the positional shift exists. FIG. 11 is a graph showing the relationship between the position in the Z direction and a light amount detected by a sensor 10 in the state A and the state B in FIGS. 10A and 10B. Curves A and C correspond to the states A and B, respectively. In FIGS. 11, Z1 and Z2 indicate measurement values of the focus positions measured in the state A and the state B, respectively, of FIGS. 10A and 10B. Between the state A and the state B of FIGS. 10A and 10B, different measurement values Z1 and Z2 of the focus positions calculated from FIG. 11 are obtained, though the focus positions are the same. That is, it is found that the measurement value of the focus position is deviated in the state B in which both the positional shift and telecentricity exist. Deviation of the measurement value never occurs as long as at least one of the positional shift and telecentricity can be set to zero. In practice, however, it is extremely difficult to zero even one of the positional shift and telecentricity. Further, even if telecentricity is zero, measurement deviation of the focus position occurs similarly if driving errors exist in the X and Y directions when the substrate stage 6 is driven in the Z direction.